In the field of offshore industry, the use of subsea trees is widespread, such trees also commonly being referred to as subsea Christmas trees. The fact is that such trees are of a complex structure, including valves, spools, fittings and the like. In general, subsea trees are intended for use at a subsea well, for example, a subsea oil well, and are adapted to be placed on top of a so-called wellhead which is designed to serve as an interface between a subsea tree and a subsea well. Subsea trees are mainly applied for controlling a flow of matter coming from the subsea well, particularly by realizing appropriate settings of valves, which does not alter the fact that other applications of subsea trees are possible as well. In any case, subsea trees may be adapted to perform one or more additional functions besides the intended main function, including injecting matter in the subsea well, realizing pressure relief, monitoring various parameters, and offering connection points for one or more devices to be used at the subsea well.
A subsea tree applied in an offshore operation is located at the sea bed, and in view thereof, a so-called remotely operated underwater vehicle is applied for operating and controlling the valves, reading indicators, etc. According to an important safety requirement, the various valves are properly tagged, wherein it is intended for the tags to be readable at all times during installation, operation and downtime of the subsea tree. In order to facilitate access of a remotely operated underwater vehicle to the various valves and other components of the subsea tree, a subsea tree is normally equipped with a so-called remotely operated vehicle (ROV) panel, which will be referred to as instrument panel in the context of this description. Thus, in a general sense, an instrument panel is designed to serve as an interface between a subsea tree and a remotely operated underwater vehicle. In particular, the instrument panel is adapted to provide access to at least an element of at least one instrument of a subsea tree, such as a control element in the form of a handle or the like, and is normally constituted by a coated steel plate which is provided with holes at appropriate positions.
When an instrument panel for arrangement on a subsea tree is at a position as intended, i.e. at a subsea position, it may happen that over time, readability of the tags is impaired, and it may even be so that access to the valves gets hindered. The reason is found in a phenomenon known as biological fouling or biofouling. Obviously, dangerous and unsafe situations may occur when the instrument panel suffers from bio fouling to such an extent that it is no longer possible to properly inspect the tags and/or control the instruments. Mechanical cleaning of instrument panels is difficult to realize due to their operating depth, which may be well beyond the reach of normal diving activities, i.e. lower than about 100 meters under the water surface. Hence, there is a need for a durable solution aimed at keeping a submerged instrument panel clean.
In general, biofouling is the accumulation of microorganisms, plants, algae, small animals and the like on surfaces. According to some estimates, over 1,800 species comprising over 4,000 organisms are responsible for biofouling. Hence, biofouling is caused by a wide variety of organisms, and involves much more than an attachment of barnacles and seaweeds to surfaces. Biofouling is divided into micro fouling which includes biofilm formation and bacterial adhesion, and macro fouling which includes the attachment of larger organisms. Due to the distinct chemistry and biology that determine what prevents them from settling, organisms are also classified as being hard or soft. Hard fouling organisms include calcareous organisms such as barnacles, encrusting bryozoans, mollusks, polychaetes and other tube worms, and zebra mussels. Soft fouling organisms include non-calcareous organisms such as seaweed, hydroids, algae and biofilm “slime”. Together, these organisms form a fouling community.
In several situations, bio fouling creates substantial problems. Bio fouling can cause machinery to stop working, water inlets to get clogged, and heat exchangers to suffer from reduced performance. Hence, the topic of anti-fouling, i.e. the process of removing or preventing biofouling, is well-known. In industrial processes involving wetted surfaces, bio dispersants can be used to control biofouling. In less controlled environments, fouling organisms are killed or repelled with coatings using biocides, thermal treatments or pulses of energy. Nontoxic mechanical strategies that prevent organisms from attaching to a surface include choosing a material or coating for causing the surface to be slippery, or creating nanoscale surface topologies similar to the skin of sharks and dolphins which only offer poor anchor points.
In the offshore industry, it is known to provide equipment with a toxic surface onto which biofouling species cannot attach and survive. Alternatively, slow release coatings can be applied. The first approach involves a release of toxic species into the marine environment, which may be prohibited in the future for obvious reasons. The second approach involves a process in which a binder resin slowly dissolves or hydrolyses such as to release a biocidally-active chemical into the immediate near-surface environment, and is expected to be prohibited and abandoned any time soon. Due to the nature of the two approaches and the harm that the bio toxic and biocidally-active materials associated therewith cause also after their release into the seawater, not only to bio fouling organisms but also to other forms of marine life, there is a need for a more environmentally friendly and green approach, which is suitable to be put to practice for the purpose of keeping submerged instrument panels clean from bio fouling.